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Extension to MIMO and Multihop Open Questions Which nodes should cooperate What (partial) interference should be forwarded How should interference be cancelled: spatially or via detection The questions apply to ad-hoc and cellular infrastructures

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DMT in MIMO Multihop Networks Quasi-static Rayleigh fading channel Channel state known only at the receivers

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DMT for Full-duplex Relays The relay can receive and transmit simultaneously The DMT for (M1,M2,M3) full-duplex system is The hop with the minimum diversity gain is the bottleneck Achieved by decode-and-forward relaying with block Markov structure Follows easily since DF achieves capacity

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Dynamic Decode-and-Forward in Half-duplex In half-duplex system, TX and RX must share time DDF introduced by Azarian et al. (IT’05) to optimize this sharing Relay listens until decoding complete, then transmit DDF achieves the best known DMT for half-duplex relay channels, yet short of the upper bound We show: Achieves optimal DMT in multi-hop relay channels Not piece-wise linear, no general closed form expression Can be cast into a convex optimization problem Extended to multiple relays

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End to End Distortion Use antennas for multiplexing: Use antennas for diversity High-Rate Quantizer ST Code High Rate Decoder Low-Rate Quantizer ST Code High Diversity Decoder We optimize the point on the DMT tradeoff curve to minimize distortion

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What about delay? Retransmissions add time diversity at the cost of delay Extends DMT to diversity-multiplexing-delay tradeoff ARQ can be done on each link and/or end-to-end. The diversity-multiplexing-delay (DMDT) tradeoff has been characterized for point-to-point links: Want to extend this to multihop networks End-to-end distortion can be optimized over the DMDT. ARQ 1 D R D R ARQ 2 ARQ 3 H1 H2H3 Infinite Queue Delay:k1 Delay:k2Delay:k3 ARQ E2E

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What about Interference Cancellation? Antennas can be used for multiplexing, diversity, or interference cancellation Cancel M-1 interferers with M antennas What metric best captures the tradeoff? Diversity/Multiplexing/SINR -1 ?

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Interference in End-to-End Distortion Interference exploitation at the physical layer improves end- to-end distortion We have proved a separation theorem for a class of interference channels Separate source and channel coding optimal We found the operating point on the DMT multihop region for minimal distortion Under delay constraints, optimization needed Investigating new notions of capacity, distortion, and separation optimality Incorporate notions of outage and expectation in capacity and end-to-end distortion Future work will apply these notions to MIMO multihop networks

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Summary and Open Questions MIMO improves MANET capacity as well as diversity- multiplexing-delay-interference cancellation tradeoffs Much room for innovation in generalized relaying and cognitive techniques for MIMO nodes Capacity and tradeoff regions still largely uncharacterized New tools for optimizing the tradeoff region operating point to maximize end-to-end performance metrics are needed Open questions in MIMO MANET design How to best use limited feedback Cross-layer design for cognitive MIMO nodes Protocol layering, separation, and interfaces Throughput Delay Diversity (T*,D v *,D l *)